Emory University's manuscript and archival collections can be found using the catalog, discoverE, or the Emory Finding Aids database. Both of these tools can be searched using keywords or linked subject headings and will indicate the holding library. Don't forget: there are several libraries with special collections materials on campus, and most do not circulate.
There are many important tools for finding archival collections relevant to your research in the United States. Some portals my be specialized around a certain topic, while others represent consortiums of holding libraries that share information.
Many countries have online tools for accessing their archives similar to the United States. Sometimes these portals are for a single country's archives, some are for a consortium of countries or special topics.